Diagnostic ultrasound ( PDFDrive )

CHAPTER 7 The Pancreas 215
A
B
FIG. 7.11 Normal Pancreatic Parenchymal Echogenicity. (A) Transverse sonogram showing pancreas has echogenicity similar to that of the
liver. (B) Transverse sonographic image of another patient, showing pancreas with a much more echogenic appearance.
because they can simulate a pancreatic head mass, a pseudomass.
Typically, these bulges extend to the right of the GDA and superior
pancreaticoduodenal artery (Fig. 7.13). Another common
pseudomass is a bulge on the anterior body, oten seen where
the let lobe of the liver touches the pancreas. Pseudomasses
have texture and echogenicity identical to normal pancreas,
allowing diferentiation from true pancreatic mass lesions. When
high-quality images of the pancreas cannot be obtained because
of technical diiculties, however, it may be diicult to diferentiate
a pseudomass from a true neoplasm of the pancreas. In these
cases, comparison to appearance on prior exam or correlation
with computed tomography (CT) or magnetic resonance imaging
(MRI) may be needed for further assessment.
Fatty Pancreas
Because the pancreatic parenchyma can be very echogenic
normally, it may be diicult or impossible to diagnose fatty
iniltration with sonography. Although the fatty pancreas has been
described as being more echogenic than the normal pancreas, 10
this is diicult to judge (see Fig. 7.12). he pancreas can appear
sonographically normal with complete fatty replacement 11 (Fig.
7.14). CT is sensitive in diagnosing fatty replacement, whereas
ultrasound is unreliable. Severe fatty replacement of the pancreatic
parenchyma can occur with cystic ibrosis, diabetes, obesity,
and occasionally, old age and Shwachman-Diamond syndrome.
Shwachman-Diamond syndrome is a rare congenital genetic
disorder characterized by pancreatic insuiciency, bone marrow
dysfunction, and skeletal abnormalities. 12
An interesting pseudomass can be caused by a relatively
hypoechoic pancreatic head or uncinate process (ventral pancreas)
compared with the dorsal pancreas (Fig. 7.15). Some evidence
suggests that this phenomenon is related to relative fatty sparing
in that part of the gland. 13-15 Based on a large prospective study,
Coulier 16 found that “hypoechoic ventral embryologic cephalic
pancreas” is never found before age 25 and is most common in
middle-aged women with a “moderately echoic pancreas.”
Embryology and Pancreatic Duct
he embryologic precursors of the adult pancreas develop as
two outpouchings (“buds”), called the dorsal (cranial) pancreatic
anlage and ventral (caudal) pancreatic anlage. hese embryonic
buds arise from opposite sides of the junction of the primitive
foregut and midgut (Fig. 7.16). he two pancreatic anlagen
(primordia) rotate to be in proximity, typically fusing at 6 to 8
weeks of gestation. 17 he dorsal (cranial) pancreatic bud becomes
the body and tail of the pancreas. he ventral (caudal) bud
becomes the pancreatic head and uncinate process, ending up
in a position caudal to the body and tail. he ventral bud is also
the embryologic origin of the gallbladder, bile duct, and liver.
he bile duct and pancreatic head sharing a common origin
explains the usual fusion (60%-80%) of the pancreatic and bile
duct in the ampulla and their common entry into the duodenum
through the major papilla.
he pancreatic duct is crucial to the exocrine function of
the pancreas, conveying the pancreatic digestive secretions to
the duodenum. Most adults have a single, main pancreatic duct
that originates when portions of the two ducts from each pancreatic
anlage fuse. he main pancreatic duct empties into the
duodenum via the major papilla, usually ater merging with the
common bile duct in the ampulla. In 20% to 40% of individuals
the ducts do not join—only the bile duct enters the duodenum
through the major papilla. 18 he pancreatic duct enters separately,
usually near the bile duct.
he duct in the body and tail (from the dorsal pancreatic
anlage) fuses with the duct in the head (from the ventral pancreatic
anlage) to form the main pancreatic duct. A portion of the duct
from the body and tail, the accessory duct (Fig. 7.17), oten
persists (≈50% in autopsy series) and enters the duodenum